The present invention relates to a method for improving the accuracy of a channel estimation process in a communications network. More specifically, it relates to estimating a channel response function of all sub-carriers in a network employing an Orthogonal Frequency Division Multiplexing (OFDM) method, based on unevenly distributed pilot signals.
Channel estimation for receivers in a wireless communications network is performed by using training signals or pilot signals. In a network employing an Orthogonal Frequency Division Multiplexing (OFDM) method, a training signal occupies all sub-carriers of an OFDM symbol while a pilot signal only uses a portion of the sub-carriers of an OFDM symbol.
Since it takes all sub-carriers to send a training signal, the network incurs high overhead and thus reduces the available channel bandwidth. In addition, training signals must be transmitted regularly, which also results in a decrease in bandwidth. Therefore, it is not practical or efficient for a network employing a burst mode transmission method for high data rate applications to use training signals for channel estimation
On the other hand, a pilot signal uses only a portion of the sub-carriers of an OFDM symbol, which makes it a better choice for channel estimation. In a burst mode transmission, predetermined pilot signals are inserted into the data stream. The predetermined pilot signals facilitate channel tracking and channel estimation for coherent detection.
Predetermined pilot signals are distributed evenly or unevenly in the time and frequency domains, i.e. the pilot signals occupy some sub-carriers at some of the times evenly or unevenly. In a wireless communications network equipped with multiple antennas, having an antenna adds another dimension for the distribution of pilot signals, i.e. the pilot signals occupy some sub-carriers at some of the times on some of the antennas.
The channel characteristics of pilot sub-carriers are correlated. The degree of correlation depends on the difference in frequency, namely the “distance” between the sub-carriers. The closer the two sub-carriers are, the more correlated their channel characteristics are. The channel characteristics of data sub-carriers are estimated based on the channel characteristics of the neighboring pilot sub-carriers.
Conventional channel estimation algorithms use training signals or evenly distributed pilot signals to compute channel characteristic information, and they may incorporate some type of decision feedback mechanism. These algorithms, however, do not guarantee accurate channel information due to the fact that pilot or training signals are often impaired by channel noise, interference, Doppler shift effect, and a frequency offset of a mobile channel. In other words, the channel characteristics are estimated with impaired signals.
As such, what is desired is a method and system for improving the accuracy of channel estimation based on unevenly distributed pilot signals for high data rate burst mode communications networks, such as (WiMAX).